The tag pipeline to a last-level cache (LLC) provides a way to access the tag, MESI, and LRU arrays. The tag pipeline (also referred to herein as a tagpipe) prioritizes requests and makes decisions about how to respond to certain requests. For instance, a load request from a lower level cache, such as the L1D, causes a queue entry to be pushed for the purposes of tracking state. The data load queue entry then arbitrates for ownership to the tag pipeline. Once it is granted ownership to the tag pipeline, the queue entry accesses the tag MESI array to see if its address is in the array. If it is, then at the end of the pipeline the queue entry decides, based on whether the address is in the cache and on what other caches have the line, whether there will be a hit or miss, or whether it will need to snoop other caches.
The logic used to arbitrate access to the LLC can result in hangs that are neither the fault of a coding error nor predictable to a programmer. For instance, deadlocks occur when a single request or pair of requests do not complete. The requests repeatedly arbitrate into the pipeline and replay. One common cause of deadlocks is a request waiting on some external stimulus. Another common cause is the existence of a dependency chain in which each one of a pair of requests is waiting for the other of the requests to complete.
Another common example is live hangs and starvations that occur when, in a multi-threaded program, each thread competes for the same shared resource. In code, ownership is often signaled by a variable that is a zero if the resource is available, and a one if the resource is not available (i.e., already owned by one of the threads). The threads set the bit to one while also reading the bit, to see if they are able to gain ownership. If a thread can read a zero but set a one, that thread now gains ownership. When one thread gains ownership, the other threads constantly do read-modify-writes to this location, waiting for the first thread to release the shared resource. Hangs occur where thread zero owns the resource, and is finished with its task, but is prevented from writing a zero to release the resource by threads one and two's repeated read-modify-writes attempting to acquire the resource. These kinds of starvation conditions are unintended features of an architecture that determines how loads are prioritized with respect to other loads and are difficult to predict.
In practice, it may be more efficient in terms of cost, processing speed, and logical complexity to create a microprocessor that detects and responds to common deadlock conditions than it is to create a microprocessor in which such conditions never or rarely occur. Accordingly, there is a need for heuristic-based tagpipe traffic monitoring logic to detect patterns indicative of a hang. There is also a need for logic that responds to detected hangs in an attempt to resolve them.